1. Field of the Invention
The present invention relates to the measurement of an optical property, such as group delay or chromatic dispersion, of a device under test (DUT).
2. Brief Description of Related Developments
A common way of measuring chromatic dispersion, e.g. defined by the dispersion coefficient D, is the so-called “Spectral Group Delay Measurement in the Time Domain”-method described e.g. in “An analysis and comparison of OTDR, photon counting and differential phase shift techniques for field chromatic dispersion measurements” by Aaron T. Deragon et al., NFOEC 2001, pp. 1585–1594. This method uses at least four or even more wavelengths generating devices at a time to measure chromatic dispersion. This method is also known as Time-of-Flight (TOF) method.
Solutions that use the TOF method are commercially available today from e.g. Anritsu Corporation or Luciol Instruments. These available prior art methods work from one end or from both ends. To illustrate the prior art methods a prior art method working from one end (single-ended) is shown in FIG. 1 and a prior art method working from both ends (dual-ended) is shown in FIG. 2. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).
According to FIG. 1 at least four pulsed light beams 1 each having a fixed but different wavelength λ1 to λn are provided being pulsed by direct modulation or using an external modulator 7.
The beams are then coupled together by a coupler 2 and send over a fiber 5 as the DUT to a fiber termination (e.g. straight connector or mirror 6) providing an end reflex, which reflects the beam to an additional coupler 3, which couples the light into a detector 4.
The additional coupler 3 is used for the single-ended measurements only. For dual-ended measurements according to FIG. 2 several light beams 1 each having a fixed but different wavelength λ1 to λn are pulsed by a modulator 7 by direct or external modulation and coupled together by a coupler 2 and send over a fiber 5 as the device under test (DUT) which emits the light direct into a detector 4.
At the detectors 4 the group delay of the fiber 5 is measured as shown in an example according to FIG. 3.
From the group delay the derivation D=1/L * dλgroup/dλ can be derived, with L being the length of the fiber 5. D is the dispersion coefficient of the fiber 5. The dispersion coefficient D is shown in FIG. 4.
It is also known in the prior art to use a broadband light source instead of several lasers and generate the different wavelengths through wavelength selective devices. The different wavelengths can be generated at the beginning or at the end of the fiber to be tested.